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I have a 280z and I know there is a height issue in the difference between the radiators, but I'm not sure if there is any other difference.
As for A/C, it normally runs off the middle pulley on a L28et. I had an unknown 16" fan in there before that was 3.5" thick and ran into clearance issues with the tensioner pulley when recently installing A/C. Now I have a SPAL fan that is ~2.5" thick and have ~7/8" of clearance. I have a Champion 3-row radiator that measures 2.75" at the top, but the core is inset from the edge of the end tank by about 3/8" and my fan is just flat up against the core (no shroud.)
*edit*
Just looked, and if you have a Champion 4-row for your 240z, you should have ~.3" less clearance than I do.

Are the grounds on the toggle switches just to light them up?
The second toggle isn't really needed. Just run those wires directly to terminal 87 on the relay. (and as you figured out, not 87A) It wouldn't hurt anything to have the second toggle though but the only benefit I could see is being able to shut the engine off while keeping the relay powered (if you have other loads also powered off this relay)
The coil does need to come off before the pushbutton (so terminal 87 of relay) but the stock wiring had two wires that went to the coil/resistor. One was energized while running AND starting (so the equivalent of before the button) and the other was energized only while starting (so the equivalent of after the button.) The one I labeled "to coil" above, was the latter. The other I labeled "Transistor Ign Unit and Coil"

The relay is energized via the toggle. That applies power to the output of the relay (Terminal 87). This would essentially be your accessory power as well as power to the coil via ignition relay. This power is stopped from going to the starter solenoid and starting wire to the coil/resistor by the push button. When you push the button those are both energized, allowing the starter to turn the engine over and fire. I have gone megasquirt, so I am not sure why it needs a second wire to the coil for starting, but I think it might be to create a hotter spark while starting.
If the toggle switch can handle enough current, you could technically just eliminate the relay in the diagram above and hook the toggle up to the wires that are shown going to terminals 30 and 87 of the relay.
Without knowing what was done to your existing wiring, I'm not sure anyone will be able to really help you. As for the switch, X3 to X4 appears to be a normally open contact, which is what you need. I don't know what X1/X2 is, but you shouldn't need it and unless it is used to illuminate the switch, you definitely do not want to put power and ground across it.
You should definitely read up on dc circuits though, you are asking questions about some very basic concepts that I would expect anyone to know if they are messing with the wiring in their car.

That's exactly how it works. I guess the only thing I failed to notice is that the line I have labeled going to Ignition relay would still be connected during starting. On a 77 this would just mean that the radio, air conditioning and wipe would still have power during cranking, whereas it cut power to them from the factory during cranking.

Here is a diagram using the wires that go to the stock ignition switch (I used a diagram for a 77, so your wire colors from the stock harness might not match up). This is the least intrusive way to accomplish what you want. If you have messed with more of your stock wiring, you may need to do some things differently though.

I think that was just a way he had it explained to him that kind of helps understand, I don't think he was truly advocating that it is scientifically accurate. A lot of guys find ways to understand why things do what they do without getting overly complex.

What type of info are you looking for? You haven't said what platform, (S30, S130, etc) or use. You are likely to get more information if you give people a better starting point from which to answer your request.
Here is something to maybe get you started. Try contacting the people who have done it directly with your questions.

One of these...
https://www.summitracing.com/ga/parts/vta-680000-vua
I used it to figure out my brackets and now I am ready to get rid of it. $20 + shipping and I'll send it your way. It comes with a brand new bracket kit and fasteners.
*SOLD*

Nope, that's basically it. However, as long as the wastegate can flow enough to perform it's function of controlling boost pressure, then how it is mounted doesn't really matter. Also, you could make up for a sub-optimal mounting position by upping your wastegate size.